Purification of gases in air may be useful or vital in enclosed environments such as spacecraft, space habitats, submarines, underground mines, and terrestrial (e.g., aircraft, armored vehicles) and non-terrestrial vehicles (e.g., pressurized rovers). In particular, people produce carbon dioxide (CO2) as a metabolic byproduct that can become noxious if accumulated within an enclosure. The average person exhales almost a kilogram (kg) of carbon dioxide per day. In environments containing higher levels of carbon dioxide, people may experience symptoms such as nausea, dizziness, and headaches. Hence, manned, enclosed environments need a mechanism to remove carbon dioxide produced within the environment. Additionally, animals and plants are sensitive to the level of carbon dioxide and would benefit from carbon dioxide control.
One proposed method of carbon dioxide control uses a new type of air separation based upon centrifugal stratification of an input air stream drawn from an enclosure's atmosphere. Centrifugal air separation separates flowing gas based on the molecular weight of the gas' constituents by passing the input gas through a tightly coiled duct. Centrifugal air separation is different than gas centrifugation which spins a quantity of input gas in a rotating cylinder to separate the gas components by molecular weight. In centrifugal air separation, the input gas flows at relatively high speed through a tightly coiled duct, causing the gas to follow the duct's coiled path (generally a helical path). As the gas travels along the coiled duct in a generally laminar manner, the input gas stratifies according to the molecular weight of the gas components. Carbon dioxide, being heavier than oxygen and nitrogen, can be separated by appropriately tapping the stratified gas stream. Generally, a centrifugal air separator may be used to separate an input gas stream into a heavy component gas stream (enriched in heavy gas components) and a light component gas stream (enriched in light gas components).
The coiled duct for centrifugal air separation generally has a small radius of curvature relative to the duct diameter and a smooth interior to reduce turbulence as the gas flows. For a coiled duct constructed of tubing, the radius of curvature of the tubing may be specified at about one times the outside diameter of the tubing. Conventionally, tube bending with a small radius of curvature (e.g., less than five times the tube diameter) risks wrinkling, cracking, and/or flattening the tube. For thin-wall metal tubing, a radius of curvature of about two times the tube's outer diameter is generally considered the limit to avoid damage to the tube integrity.
Hence, there is a need for systems and methods to form tightly coiled ducts for centrifugal air separation which can form the ducts without significant wrinkling, cracking, or distortion.